The present invention relates to three-phase AC motor drives and particularly to a terminator for power lines communicating between such motor drives and a connected AC motor.
Common AC induction motors use three-phase electrical power connected to stator windings of the motor. Each stator winding receives a different conductor of a three-phase power source, in which each conductor communicates an AC power waveform shifted with respect to the other conductors by plus and minus 120°.
The three-phase power source to which the AC motor is connected may be line power or a solid-state motor drive connected to line power and synthesizing new three-phase power to drive the AC motor. This synthesized three-phase power, for example, may have a different frequency than line power to provide for motor speed control or control of other motor parameters.
The motor drive may communicate with the AC motor by means of a power cable extending hundreds or even thousands of feet. Such power cables normally include three power phase conductors and one or more ground conductors within a conductive shield, the latter to reduce the transmission of electrical interference to surrounding equipment.
The distributed inductance and capacitance of the power cable can create electrical reflections along the power cable, reflections occurring at points of impedance mismatch between the characteristic impedance of the power cable and the motor and/or the drive. These reflections produce voltage and current surges that can damage insulation on the power cable or motor, cause arcing across motor bearings, and boost the back-voltage to the motor drive damaging it or causing it to shut down.
U.S. Pat. No. 5,831,410 assigned to the assignee of the present invention and hereby incorporated by reference, describes a terminator that may be attached to the conductors of the power cable to significantly decrease reflection and thus voltage and current surges. This terminator provides a series capacitor and resistor connected between each of the three conductors to provide impedance matching between the motor and the power line thereby reducing reflection.
The present inventors have recognized that there are, in fact, two different impedances associated with the power cable and, as a result, two different reflection modes. The first reflection mode is related to the cable's common-mode impedance while the second reflection mode is related to the power cable's differential-mode impedance. The prior art terminator reduced differential-mode reflection (and hence transients) while still permitting common-mode reflections and transients.
These latter common-mode voltage transients can cause premature bearing failure by creating a condition for arcing across the bearings, pitting the finely machined bearing surfaces. Common-mode voltage transients also contribute to other transient related problems, including the transmission of electrical interference to surrounding equipment.
These two modes of reflection in the power cable have also been recognized in the paper, “Transmission Line Effects on Motor Feed Cables: Terminator Design and Analysis in the Laplace-Domain” by B. Bolsens et al. (2003). In this paper, Bolsens proposes a terminator electrically similar to that described in the above U.S. Pat. No. 5,831,410 but employing a “wye” rather than a “delta” configuration, with the important addition of a “common-mode” resistive matching element extending between a common tie point for each of the impedance matching elements (series resistors and capacitors connected to each of the conductors) and ground to reduce common-mode reflection.